Ferritic stainless steel composed of an Fe—Cr—Al-based alloy has excellent oxidation resistance at high temperatures and, therefore, is used in members (e.g., catalyst carriers, a variety of sensors, and the like) for automotive exhaust gas purifying devices for automobiles, motorcycles, marine bikes, motorboats, and the like. Furthermore, it is used in members of frames for stoves, gas burners, and heating furnaces, and also is used in electric heating elements of heaters by taking advantage of its property of high electric resistivity. For example, Japanese Unexamined Patent Application Publication No. 56-96726 discloses a metal honeycomb using, instead of a conventional ceramic catalyst carrier, an Fe—Cr—Al-based stainless steel foil with a foil thickness of 20 to 100 μm having excellent oxidation resistance at high temperatures for the purpose of downsizing of catalyst carriers for automotive exhaust gas purifying devices for automobiles and improvement in engine performance. Regarding this metal honeycomb, for example, flat stainless steel foils (flat sheets) and corrugated stainless steel foils (corrugated sheets) are alternately stacked to form a honeycomb structure, and a catalytic material is applied onto the surface of the stainless steel foils. The resulting metal honeycomb is used for an automotive exhaust gas purifying device. FIG. 1 shows an example of a metal honeycomb. A metal honeycomb 4 is produced by winding a stacked structure including a flat sheet 1 and a corrugated sheet 2 into a roll shape, and fastening the outer periphery thereof with an external cylinder 3.
When the wall thickness of a catalyst carrier is decreased by using such a stainless steel foil, its thermal capacity is decreased. Consequently, the catalyst can be activated in a short period of time from engine starting, and exhaust back pressure can be decreased, thus being effective in downsizing of the catalyst carrier and improvement in engine performance.
On the other hand, automobile exhaust gas regulations are expected to be further tightened from the standpoint of environmental protection. To reduce nitroxides, carbon monoxide, and carbon hydrides emitted from gasoline vehicles and the like, techniques such as one in which a catalyst carrier for an automotive exhaust gas purifying device is placed at a position immediately below the engine, close to the actual combustion environment, and a catalytic reaction is caused by high-temperature exhaust gas with the result that the amount of harmful substances in exhaust gas is reduced, have been developed. Furthermore, to improve fuel efficiency, engine combustion efficiency is enhanced, and the temperature of the exhaust gas itself is increased. Accordingly, catalyst carriers for automotive exhaust gas purifying devices are likely to be subjected to strong vibration in a harsher environment than before.
Stainless steel foils imparting various properties to catalyst carriers for automotive exhaust gas purifying devices have been proposed to cope with such a situation. For example, Japanese Unexamined Patent Application Publication (translation of PCT Application) No. 2005-504176 discloses a heat-resistant stainless steel foil in which Mo and W are incorporated into an Fe-20Cr-5Al alloy containing Zr, Hf, and a rare-earth element, and 1% to 15% of Ni is further incorporated thereinto to precipitate NiAl and, thereby, proof stress (strength) at high temperatures is increased, resulting in improvement in durability. Furthermore, Japanese Patent No. 3210535 (Japanese Unexamined Patent Application Publication No. 8-168680) discloses a stainless steel foil having excellent strength at high temperatures which is a material for low thermal capacity and low exhaust gas pressure use, in which the foil thickness is less than 40 μm, and the Al content and the Cr content are changed in response to the foil thickness, and into which Nb, Mo, Ta, W, and the like are incorporated. Japanese Unexamined Patent Application Publication No. 5-277380 discloses an alloy foil for a metal carrier in which proof stress at high temperatures and oxidation resistance are improved by incorporating Nb, Ta, Mo, and W into an Fe-20Cr-5Al alloy containing Y Mischmetal or mixed metals. Furthermore, Japanese Examined Patent Application Publication No. 6-104879 discloses a heat-resistant stainless steel foil in which proof stress at high temperatures are enhanced and durability is improved by incorporating a predetermined amount of Ta relative to C+N, Mo, W, and Nb into an Fe-20Cr-5Al alloy containing La, Ce, Pr, and Nd.
However, since a large amount of Ni, which is an austenite-stabilizing element, is contained in the stainless steel foil described in JP '176, the amount of solid solution Al, which is a ferrite-stabilizing element in steel, is decreased in the foil oxidation process, and part of the ferrite is austenitized. Therefore, the thermal expansion coefficient is largely changed, and strength at high temperatures, in particular, distortion resistance, of the catalyst carrier is decreased, which is a problem. As a result, in some cases, failures may occur such as falling off of the carried catalyst and collapse of the corrugated sheet.
Furthermore, in the stainless steel foils described in JP '535, JP '380 and JP '879, incorporation of Nb and Ta improves strength at high temperatures, but markedly decreases oxidation resistance at high temperatures, which is a problem. Moreover, Nb and Ta are likely to form oxides with Fe and Al, and such oxides cause distortion of the foil geometry in the heating process and the cooling process (a decrease in strength at high temperatures).
Furthermore, when the stainless steel foils described in JP '176, JP '680, JP '380 and JP '879 are applied to catalyst carriers for automotive exhaust gas purifying devices for marine bikes and motorboats, salt corrosion is likely to be caused by contact with seawater or the like, which is also a problem.
It could therefore be helpful to provide a stainless steel foil having high strength at high temperatures, excellent oxidation resistance at high temperatures, and excellent salt corrosion resistance, and a catalyst carrier for an exhaust gas purifying device using the foil.